DISCUSSION We have formalized the dynamics of CNS using Price's 17, 18 equation of evolutionary genetics. This equation provides a standard approach for capturing selection arguments within evolutionary biology and beyond 25-28. We have also formalized the idea of the universe being a purposeful object—with the design objective of black-hole formation—using an optimization program, which is a standard approach for capturing the notions of purpose, goal, or agenda 19, 22, 29. Finally, we have established formal connections between these two mathematical objects, confirming that CNS acts as if according to a design objective of black-hole formation. Insofar as CNS is an important driver of the evolution of the multiverse (and we make no claim that it is), successive generations of universes will appear increasingly well designed to produce black holes.
This approach mirrors the way in which ideas of selection and design are formalized and connected in evolutionary biology, that is, the theory of Darwinian adaptation 30. Specifically, the idea of natural selection driving genetic change of biological populations is formally captured using Price's equation 17, 19, 22, 25-28, 31; the idea of individual organisms appearing designed to maximize their Darwinian fitness is formally captured using an optimization program 19, 22, 29; and the connection between these ideas is formalized by deriving correspondences between these two mathematical objects, translating dynamics into optimization and vice versa 19, 22, 29. These mathematical results provide formal license to use intentional language in evolutionary biology: for example, selfishness, altruism, and conflicts of interest 32. This analogy of intentionality not only provides a powerful shorthand that can be translated with fidelity into statements about gene frequency dynamics, but it also defines whole programs of scientific research: for example, parent–offspring conflict and the evolution of altruism (reviewed by Gardner 33).
The theory of CNS was developed as an alternative to the observer-bias explanation for apparent fine-tuning of the fundamental constants of nature, that is, the weak anthropic principle 34. This holds that we should be unsurprised by the apparent contrivance of our universe for the purpose of supporting intelligent observers, given that an alternative universe that could not support intelligent observers would not be observed. Thus, the CNS versus observer bias hypotheses concerning the apparent contrivance of the cosmos mirror the Darwinian versus Cuvierian approaches to explaining biological adaptation: although Darwin described a mechanical process that drives the evolution of adaptation, Cuvier suggested that nonadapted organisms, being unable to survive and reproduce, would not be observed, and so our observations of adapted organisms require no special explanation (reviewed by Reiss 35). Whilst the observer bias hypothesis for apparent cosmological fine-tuning has some predictive power, it does not explain why the universe is fine-tuned and observed rather than not fine-tuned and not observed. Indeed, proponents of the observer-bias view have argued that a full explanation is only achieved by invoking a large multiverse, so that at least one universe appears sufficiently fine-tuned to support intelligent observers (see, e.g., Ref. [ 11 or 36). Thus, the notion of a multiverse is central to both the CNS and the observer-bias hypotheses. One potentially desirable feature of the CNS approach is that it removes the observer from the explanation, thereby achieving greater objectivity 37.
CNS differs from biological natural selection in a number of respects. For example, although mortality and competition for resources are basic facts of biological populations, they are entirely absent in the CNS model of the evolving multiverse. These differences have been used to argue that CNS is only weakly analogous to Darwinian natural selection 16. However, Price's 17, 18 equation captures the essence of selection occurring in any medium, whether cosmological or biological, and it emphasizes that neither mortality nor resource competition are fundamental aspects of selection. Rather, natural selection is the part of change attributable to the covariance between heritable characters and fitness. By framing the CNS hypothesis in terms of Price's 17, 18 equation, we have clarified the fundamental analogy between CNS and Darwinian natural selection.
However, the theory of CNS does differ from Darwinism in three important respects. First, Darwinism was developed as an explanatory framework that could account for apparent design in the biological world, invoking only phenomena whose existence was beyond reasonable doubt; for example, the Malthusian struggle for existence and the heritability of organismal characters 30. In contrast, the theory of CNS invokes speculative ideas; for example, a multiverse and successive generations of universes that inherit their fundamental constants from their parents, without any evidence for the existence of either phenomenon 12-14. However, given the importance of selection-like processes for generating apparent design in the natural world, it is arguably sensible to seek a selection-like explanation for the apparent design of our universe 13. Second, Darwinism yields readily testable predictions, with the diversity of living organisms providing swathes of data against which these predictions may be tested. In contrast, there is only one visible universe against which the predictions of CNS may be tested. However, Smolin 13 has outlined a number of falsifiable predictions made by the theory of CNS, including a reasonably specific upper limit for the mass of neutron stars (see also Refs. [ 37 and 38).
Third, although evolutionary arguments typically involve transformations through a well-defined concept of time, this is not true of Smolin's 12-14 CNS hypothesis. Cosmologically, there is no meaningful notion of absolute time even within a single universe: relativity teaches us that there are rather many equally valid time-slicings (technically space-like foliations). This problem is exacerbated by multiple universes. Happily, Price's 17, 18 equation is sufficiently versatile that it can be applied to transformations occurring between any two populations, irrespective of how these are temporally related. Following Smolin 12, we have focused on transformations between generations, where each universe is assigned to the generation immediately subsequent to that of its parent. Although this makes the question of formalizing CNS well posed, the physical meaning of this between-generation transformation remains very unclear.